Explain why the curdling of milk is a chemical change.

Curdling of milk is a chemical change because milk proteins (mainly casein) undergo irreversible chemical reactions—they denature and coagulate due to acid or enzymes—forming new substances with new properties (curd and whey).

Fresh milk is a complex colloid of fat globules and proteins (casein micelles) in water with lactose and minerals. When we add an acid (like lemon juice or vinegar) or introduce lactic acid bacteria (as in yogurt making), the pH of milk drops. As acidity increases, the casein micelles lose their charge stability and denature. They then link together into a network, coagulating into soft white clumps called curd, leaving behind a watery liquid called whey.

This process creates new material properties: the curd is solid or semi-solid, tangy, and behaves very differently from liquid milk. The change is not easily reversible; you cannot turn curd back into fresh milk by simple physical means like cooling or stirring. That irreversibility is a hallmark of a chemical change. Often, there is also growth of bacteria that convert lactose to lactic acid, further confirming that new substances are formed.

Enzymes like rennet (chymosin) can also curdle milk by cutting specific bonds in casein, again a chemical action. Heat can trigger curdling too (especially in acidic sauces), because high temperatures denature proteins. In all these routes, the essential feature is the same: the original molecules change their structure and interactions forever. That sets curdling apart from physical changes like melting ice or dissolving sugar, where no new substances are created.